1. Field of the Invention
This invention relates to subscriber loop carrier systems and, more particularly, to coin station signaling in such systems.
2. Description of the Prior Art
It is well-known to provide a large number of telephone subscriber channels on a single transmission facility by means of time division or frequency division techniques. A time division subscriber loop carrier system utilizing delta modulation is shown in R. J. Canniff et al U.S. Pat. No. 4,048,448, granted Sept. 13, 1977.
Subscriber loop carrier systems of the type disclosed in the above-identified Canniff et al patent are arranged to provide standard telephone supervisory signaling through the multiplex transmission system. Thus, off-hook supervision and ringing are provided by utilizing special codes in the digital pulse stream to carry the supervisory information. If such a system is to be used for providing service to coin stations, it is necessary to similarly encode all of the necessary coin station supervisory signals such as coin check, coin collect and coin return as well as the normal off-hook and ringing supervision. Since this type of supervision utilizes a wide variety of direct current voltages and polarities at the central office, it is necessary to detect these supervisory signals, encode them for transmission on the multiplex transmission facility and regenerate the supervisory conditions at the remote coin station drop wires. The large number and complex interrelationship of these supervisory signals have made the provision of coin station service in subscriber loop carrier systems a difficult objective.
Coin stations can provide service in two different modes. The first mode, called "dial tone first," enables the station user to receive dial tone immediately upon picking up the handset and hence the user is immediately able to dial the called number. Some coin stations, however, operate in a "coin first" mode in which the user must drop a coin into the pay station slot before receiving dial tone and thus being enabled to dial a number.